The "No value selected" radio shows up for DataDrivenEnumeration when the field is NOT required or if it doesn't have a defaultValue set. You can use metadata overrides to set a default value for this field or make it required. This is driven by BasicFieldMetadata#getAllowNoValueEnumOption().

Alternatively, you could set the following property which will cause all DataDrivenEnumeration fields to render as drop-downs.